On 20/08/2011 06:48, Oli Glaser wrote:
> As far as I understand it, THD can never be *higher* than THD+n, as=20
> THD+n is THD with all the noise in between the harmonics included.

Assuming you are at the same signal level and conditions...

The only way to "measure" THD is to measure noise, then measure THD+n=20
and then make assumptions that you can simply subtract the 1st reading=20
in dB from the 2nd one. But the impedance, shielding all conditions must=20
be the same.

Note an unterminated input or signal generator that's connected but=20
"off" can be noisier than or quieter than the generator on with no signal.

Maybe you need to compare with a 20dB pad on the input. Then change in=20
noise level of generator vs its on/off and drive level will be attenuated.

Without exact diagram of your set up it's hard to say what is happening.


--=20
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